1. Field of the Invention
This invention relates to the removal of molten polymer from a polymer mixer. More specifically, this invention relates to the removal of molten polyethylene from a continuous polymer mixer.
2. Description of the Prior Art
Although, for sake of clarity and brevity, this invention is described in terms of conveying molten polyethylene, this invention is not limited to that type of polymer.
Ethylene is polymerized to polyethylene homopolymers and co-polymers by a number of different processes to make different polymeric products such as low density polyethylene, high density polyethylene, and linear low density polyethylene which exhibits favorable characteristics found in both low density and high density polyethylene. For sake of example only, this invention is described herein primarily in terms of a slurry phase (suspension) polymerization process for making high density polyethylene (HDPE).
The slurry polymerization process typically takes place in a closed loop (horizontal or vertical) reactor using a hydrocarbonaceous solvent such as n-hexane, isobutane, isopentane, and the like. The essentially liquid feed mixture of ethylene, co-monomer(s), if any, catalyst, and any additives is continuously pumped in a loop while the polymerization reaction takes place.
The process can employ known catalyst systems such as a silica-supported chromium/aluminum catalyst with or without a co-catalyst such as triethyleborane, or Ziegler-Natta catalyst systems comprised of titanium tetrachloride/trialkyl aluminum, or other transition metals such as zirconium and vanadium in place of the titanium. These catalyst systems are well known in the art and more detail is not necessary to inform one skilled in the art.
While the aforesaid feed mixture is continuously circulated in the loop reactor, polymerization takes place at temperatures below the melting point of the polyethylene formed thereby producing a slurry of solid polyethylene particles in the liquid feed mixture. The reaction typically takes place at a temperature of from about 185 to about 220 degrees Fahrenheit (F.) at a pressure of from about 500 to about 650 psig. A slurry containing, among other things, HDPE and solvent is drawn off from the reactor either continuously or intermittently, as desired.
The loop reactors are normally formed from large diameter pipes, e.g., from about 10 to about 30 inches in inside diameter, and can be about 50 feet across with lengths of from about 250 to about 300 feet in length.
The slurry withdrawn from the reactor is processed for the removal of solvent for re-use in the reactor. The remaining solid polyethylene particles are then passed to a drying, mixing, extruding, and pelletizing system wherein the particles are converted to solid polyethylene pellets. The pellets are packaged and marketed as a product of the polyethylene production plant in which the foregoing process was carried out.
A high shear mixer such as a commercially available fifteen inch Farrell continuous mixer has been employed in this system. The operation of the mixer unit is to receive solid polyethylene powder at a temperature of from about ambient to about 140 F, and to mix this powder until the mixing action raises the temperature of the powder to a temperature of from about 360 to about 420 F, thereby melting the powder and forming a stream of molten HDPE.
The molten polymer is removed through an outlet orifice gate carried by the mixer and transferred through a chute/hopper conduit combination to an extruder unit in which the molten polymer is extruded as a first step toward making solid polymer pellets suitable for storage, packaging, and the like.
It is the orifice gate of the mixer in the drying, mixing, extruding, and pelletizing system to which this invention is directed. The extent of the mixing undergone by the polymer in the mixer determines the temperature of the polymer when it leaves the mixer and enters the outlet gate orifice.
Heretofore, the hinged gate inside the outlet orifice of the mixer carried a clevis at its lower end. This clevis was internally threaded and thereby connected to a threaded shaft end, the opposing end of this shaft being connected to an actuator.
Operation of the actuator moves the hinged orifice gate backward or forward, as desired.
Movement of the gate backward gradually opens the orifice further, thereby allowing a greater volume of polymer to leave the mixer through the outlet orifice. This shortens the residence time for the polymer in the mixer, and lowers the temperature of the polymer exiting the outlet orifice.
Movement of the gate forward gradually closes the orifice further, thereby allowing a lesser volume of polymer to leave the mixer through the outlet orifice. This retains the polymer in the mixer for a longer mixing time, and thereby raises the temperature of the polymer exiting the outlet orifice.
Thus, the orifice gate of the mixer is used to vary, as and when desired, the melt temperature of the polymer leaving the mixer. The orifice gate/actuator combination affords an infinite number of gate settings that control the amount of molten polymer leaving the outlet orifice in which the gate is movably carried. Accordingly, great flexibility is available in achieving the desired melt temperature of the polymer exiting the outlet gate orifice of the mixer.
In actual operation, frequent failure of the actuator shaft at its threaded end was experienced. This failure required that the polymer mixer be shut down, the mixer opened, the broken threaded portion in the lower part of the gate drilled out, and a new threaded shaft installed in place of the failed shaft. This repair work usually translated into 14 to 24 hours of mixer downtime and lost mixer production, an expensive loss.
Surprisingly, it was found that even though the actuator shaft was carried essentially horizontally between the actuator and the orifice gate, when the actuator was operated to move the gate forward, a net downward force was exerted on the threaded end of that shaft which caused the frequent failures of this type of shaft.
This invention addresses and corrects the failure mode of the aforesaid threaded actuator shafts.